Embedded-atom study of grain boundary segregation and grain boundary free energy in nanosized iron–chromium tricrystals

Abstract

We present a theoretical study of equilibrium grain boundary (GB) segregation in a tricrystal setup using a thermodynamically accurate iron–chromium embedded-atom potential. Through continuous variation of the chemical potentials, the full concentration range is explored in the temperature range of 600 K–1100 K, evaluating segregation below and above the critical temperature of the miscibility gap. Key findings are: i) the GB excess entropy is rather small and shows only weak variation with temperature; ii) due to the small lattice mismatch in the iron–chromium system, segregation can reasonably be predicted by comparing the T=0 energies of iron and chromium; iii) every atomic site in the defects undergoes the occupation probability transition at a different chemical potential, resulting in stepwise segregation isotherms; iv) the extraordinary thermodynamic properties of the iron–chromium system lead to negative GB free energies on the chromium-rich side; v) the width of the GB segregation zone decreases with temperature and depends significantly on the bulk concentration; vi) the additional triple junction (TJ) excess segregation is small relative to the GBs and for the most part a geometric effect of the finite width of the GB segregation zones. Available experimental data of GB segregation on the iron-rich side are in good quantitative agreement with the presented results.